1. Field of the Invention
In electrowinning processes, the purity of the electrolyte used depends on the quantity of impurities present in the material from which the metal to be deposited on the cathode and solubilized during the leaching process is obtained.
2. Description of the Background
In processes based on sulphur-containing electrolytes, owing to the low cost of the electrolyte itself, the electrolyte may be discharged in the water treatment plants usually present in the plant, resulting in a significant loss of electrolyte.
Owing to its high cost and the difficulty of processing it in a water treatment plant, the fluoboric electrolyte cannot undergo the same cycle as the sulphur-containing electrolyte.
For this reason, in the technical field in question attempts are being made to find a method which allows the elimination of these impurities, while minimizing at the same time the loss of electrolyte.
The most important fluoboric electrowinning processes are associated with the recovery of Cu and Pb from minerals or from by-products where they are present together with other elements which are partially or totally dissolved during the leaching step. In this connection Swiss Patent Application No. 1886/97 in the name of the same Applicant may be mentioned.
The more noble impurities of the metals to be deposited are usually removed by means of cementation, i.e reduction to the metal by the metal to be deposited in accordance with the reactions: EQU Pb+Me.sup.n+ =&gt;n/2Pb.sup.2+ +Me
(where Me=Ag, Cu, Sb, Bi, etc.) EQU Cu+Me.sup.n+ =&gt;n/2Cu.sup.2+ +Me
(where Me=Ag, Bi, etc.)
The less noble impurities of the metals to be deposited, such as Fe, Zn, Ca, Mg, Cd, etc., are much more difficult to remove and, if they are not removed, tend to accumulate in the solution, reducing the quantity of free acid. This causes two types of problem: a reduction in the dissolving power of the solution during the leaching step and a reduction in the fineness of the crystal grain of the deposited metal. For these reasons it is very important to find a method which allows the removal of these types of impurities, while keeping the acidity in a suitable concentration range, and this is the main object of the present invention.